Typically, mobile devices offer the possibility to record video and audio data. For a spatially extended audio experience, some mobile devices even allow the audio data to be natively recorded as surround sound using multiple microphones and substantial post-processing of the microphone signals. Conventional mobile devices like smart phones and tablets, however, do not provide the capability to record such multi-channel surround sound, because for conventional surround sound recording techniques, large and expensive microphone arrays or setups are required.
For example, augmented DECCA Tree, Optimized Cardioid Triangle (OCT) and XYtri configuration are known as a setup for surround sound recording. Because of their size, these setups are not applicable for mobile devices.
More compact conventional microphone setups also known for surround sound recording are, for example, the “Soundfield microphone” (as described by K. Farrar, “Soundfield microphone: Design and development of microphone and control unit”, Wireless World, pages 48-50, October 1979) and the “Schoeps Double MS” (as described under http://www.schoeps.de/en/products/categories/dms). However, both setups require the use of specific pressure gradient microphone elements, which are not suited for rather small mobile devices like tablets, smartphones or the like.
Some approaches in the other approaches use omnidirectional microphones for recording sound, where the advantage is that cheap microphones can be used. For instance, a pair of omnidirectional microphone signals can be converted to two first-order differential signals to generate a stereo signal with improved left-right separation (as described, for instance, by C. Faller, “Conversion of two closely spaced omnidirectional microphone signals to an xy stereo signal”, Preprint 129th Conv. Aud. Eng. Soc., November 2010). However, a weakness is that the differential signals have a low signal-to-noise ratio (SNR) at low frequencies, and have spectral defects at higher frequencies. This effect strongly depends on the distance between the microphones. At small distances, also low frequencies are affected. The distance between the microphones for recording front/back signals is limited by the thickness of the device when recording sound using a mobile device such as a tablet. As modern devices are typically less than one centimeter thick, the maximum distance between the microphones is small. In this case a front/back separation is not sufficiently resolved, and consequently no surround recording is possible for small setups. That is, for these approaches still a large spacing between the microphones is needed.
Some other approaches use directional microphones (e.g., cardioid) for surround sound recording. The advantage is that the microphones can be placed close to each other (co-incident). However, more complex and expensive directional microphones are required.
Generally, it is technically difficult due to the small form factors of mobile devices to arrange microphones that capture good surround sound, because the recording of surround sound requires a number of microphones with specific placements and directional responses. Additionally, surround sound recording typically requires expensive directive microphones. Such directive microphones are also required to be mounted in free air, but on mobile devices only one sided openings are possible, which limits the use of sound pressure (i.e. omnidirectional) microphones.
As a result of the above, in the existing market only a few mobile devices, namely high-end dedicated video cameras, which are typically big and expensive, feature surround sound recording. Smaller mobile devices, like smart phones and tablets, usually feature only mono or limited stereo sound capture. There is a need for suitable small and cost-effective microphone setups, for example for portable devices like tablets or smartphones.